I am using a 160° high flow thermostat right now. I have a six blade fan with a 409 fan shroud I am running about 60% water 40% antifreeze I am using a clutch fan that has been tested that was original to the car. The motor has about 2000 miles on it when I did the motor. I also rebuilt the water pump.Are you using a thermostat? If so, what temperature is it rated at? The reason I ask is a thermostat will provide a restriction, slowing the coolant flow. Also, what ratio of water to antifreeze? Too much antifreeze won't dissipate the heat as well. Are you running a fan shroud? What fan are you running, with or without a clutch?
Next question I would have, when is the car running hot or overheating? Probably should have asked previously. I don't have any experience with high flow thermostats, I run a regular thermostat. I believe it is a 160 degree, cheap two core aluminum, fan shroud, 18" six blade clutch fan, 409 shroud and the engine runs 175 degrees while moving and may get up to 210 stopped in traffic on a hot day. Immediately cools back down to 175 once I am moving again. Pretty much the same set up as you have except the radiator and high flow thermostat.I am using a 160° high flow thermostat right now. I have a six blade fan with a 409 fan shroud I am running about 60% water 40% antifreeze I am using a clutch fan that has been tested that was original to the car. Some motor has about 2000 miles on it when I did the motor. I also rebuilt the water pump.
No a/c on my Impala.You have a good point about the thermostat. I used it with a different radiator I had and it seemed to work for that one. I have not tried to go back on the smaller slow thermostat. Do you have AC on your car cause I have AC and I have the big condenser in the front of the radiator. I am pretty sure if I didn’t have the condenser it would be fine. But I am trying to keep the A/C but it may have to go if I can’t figure it out.